The only known method for measuring the content of unburnt coal in the ash is to perform a chemical analysis of the ash itself. This method is often unsuitable because in most cases the parameter would have to be known in real time, that is as the ash is being formed. On the other side, very often, the knowledge of said parameter is not requested with an high degree of accuracy as attainable through the chemical analysis methods.
Anyway, it is known that the dielectric constant of a dielectric material is a function of the content of conductive particles in the material: the higher the conductive particles concentration, the higher will be the dielectric constant of the material, both as far as its real part and imaginary part are concerned.
In particular the coal ash without unburnt coal has the characteristic of a substantially perfect dielectric material, i.e. with a very low value of the imaginary part of the dielectric constant.
On the contrary, unburnt coal has a conductivity substantially other than zero. Therefore, the presence of more unburnt coal in the ash increases the dielectric constant, and the unburnt concentration is greater. The increase affects both the real part and the imaginary part of the dielectric constant; thus, in order to have a thorough evaluation of the phenomenon, in the following reference will be made to the modulus of the dielectric constant, except when specified to the contrary.
The present technical knowledge teach that a biunivocal correspondence exists between the concentration of unburnt coal in the ash and the dielectric constant of the ash itself. The theoretical study of the problem with the purpose of making analytically explicit such a biunivocal correspondance, is not necessary. What is necessary is only a calibration curve which can be easily obtained from several samples of ash characterized by known unburnt concentrations measured, for example, by chemical analysis.
The calibration curve being known, the technical problem is therefore how to measure the ash dielectric constant in a quick and easy way. Unfortunately the methods and apparatuses that have been proposed up to now, involve the use of samples of well defined shape and also are unsuitable for characterizing materials having comparatively high losses.